1. Field of Invention
The present invention relates generally to control circuits for programmable frequency synthesizers. More specifically, the present invention relates to a control circuit for a programmable frequency synthesizer utilizing a voltage controlled oscillator circuit.
2. Description of Related Art
Voltage controlled oscillators are widely used in communication devices such as cellular telephones to generate oscillatory signals. In particular, a voltage controlled oscillator is one of the components of a phase-locked loop or PLL, which is an electronic circuit used to generate a stable oscillatory signal having a desired frequency of oscillation. Such circuits are commonly referred to as frequency synthesizers. The signal output by a PLL frequency synthesizer may be used, for example, as a carrier signal in a transmitter or as a local oscillator signal in a receiver.
In order to tune a transmitter or receiver to a particular frequency, it is necessary to alter the frequency of oscillation of the carrier or local oscillator signal in a controlled manner. This can be achieved by synthesizing the carrier and local oscillator signals using a programmable frequency synthesizer including a VCO. A VCO is tuned by varying an input DC control voltage level.
Most portable electronic devices use a five-cell or four-cell battery to generate DC power. Devices having five-cell and four-cell batteries can generate a regulated DC power supply voltage level of about 4.8 volts and 3.8 volts, respectively. In the case of a five-cell battery, the DC voltage control signal VC used as a control input to the VCO has a range of between about 0.7 and 4.3 volts, or approximately 3.6 volts. In the case of a four-cell battery, VC has a range of between about 0.6 and 3.0 volts, or 2.4 volts. The measure of the frequency responsiveness of a VCO with respect to input voltage VC is known as the gain of the VCO, and is measured in megahertz per volt (MHz/V). Thus, if the input control voltage has a lower range, the VCO must have a greater gain in order to be tunable over the same frequency range.
To be useful in an RF communication device such as a dual band cellular telephone, a voltage controlled oscillator must be tunable over a range of approximately 90 MHz for some applications. Thus, a VCO for use in a device having a five-cell supply must have a gain of about 26 MHz/Volt. A VCO for use in a device having a four-cell supply must have a gain of about 40 MHz/Volt.
One drawback to a VCO is the fact that it produces a certain amount of phase noise, which can degrade the performance of a communication system. A VCO must be designed such that phase noise is kept within certain predetermined limits. In a voltage controlled oscillator, phase noise is directly proportional to the gain of the oscillator. This is due to the fact that the varactor diode in a VCO exhibits a higher internal series resistance when a low control voltage is applied across its terminals. This resistance decreases the quality factor of the oscillator's resonant circuit, leading to increased phase noise.
There is currently a trend in the communication industry to design and produce electronic devices capable of operating with a three-cell battery. A three-cell battery provides a control voltage range of from about 0.5 to 2.2 volts, or 1.7 volts. A conventional VCO having a gain corresponding to such a control voltage range would exhibit unacceptable phase noise characteristics. Thus, there is a need for a voltage controlled oscillator and related control circuitry which exhibits acceptable phase noise performance in a device having a low supply voltage level relative to that normally used for the operation of a voltage controlled oscillator in a battery powered RF communications device, for example.